Navigating the fungal battlefield: cysteine-rich antifungal proteins and peptides from Eurotiales

Abstract

Fungi are ubiquitous in the environment and play a key role in the decomposition and recycling of nutrients. On the one hand, their special properties are a great asset for the agricultural and industrial sector, as they are used as source of nutrients, producers of enzymes, pigments, flavorings, and biocontrol agents, and in food processing, bio-remediation and plant growth promotion. On the other hand, they pose a serious challenge to our lives and the environment, as they are responsible for fungal infections in plants, animals and humans. Although host immunity opposes invading pathogens, certain factors favor the manifestation of fungal diseases. The prevalence of fungal infections is on the rise, and there is an alarming increase in the resistance of fungal pathogens to approved drugs. The limited number of antimycotics, the obstacles encountered in the development of new drugs due to the poor tolerability of antifungal agents in patients, the limited number of unique antifungal targets, and the low species specificity contribute to the gradual depletion of the antifungal pipeline and newly discovered antifungal drugs are rare. Promising candidates as next-generation therapeutics are antimicrobial proteins and peptides (AMPs) produced by numerous prokaryotic and eukaryotic organisms belonging to all kingdom classes. Importantly, filamentous fungi from the order Eurotiales have been shown to be a rich source of AMPs with specific antifungal activity. A growing number of published studies reflects the efforts made in the search for new antifungal proteins and peptides (AFPs), their efficacy, species specificity and applicability. In this review, we discuss important aspects related to fungi, their impact on our life and issues involved in treating fungal infections in plants, animals and humans. We specifically highlight the potential of AFPs from Eurotiales as promising alternative antifungal therapeutics. This article provides insight into the structural features, mode of action, and progress made toward their potential application in a clinical and agricultural setting. It also identifies the challenges that must be overcome in order to develop AFPs into therapeutics.

Keywords: antifungal mode of action; antifungal proteins and peptides; antifungal resistance; fungal infection; fungal pathogens; structure-function relation.

Figure 1

The role of fungi in agriculture, industry, health and environment. Schematic overview of the multifunctional roles of fungi in the agricultural, industrial, environmental and health sector. Created with BioRender.com.

Figure 2

Fungal diseases in humans and plants. Schematic overview of fungal infection sites in (A) humans and (B) plants. Examples of human or plant pathogenic fungi are grouped by their predominant manifestation site. Created with BioRender.com.

Figure 3

Antifungal drug classes used against fungal pathogens in the health and agricultural sector. The six classes of antifungal agents used in the agricultural sector are morpholines, anilinopyrimidines, strobilurins, succinate dehydrogenase inhibitors, benzimidazoles and azoles (green). Examples for clinically approved antifungal classes are azoles, polyenes, echinocandins and nucleoside analogs (red). Created with BioRender.com.

Figure 4

Representativ AFPs of fungal origin. Exchange (PDB: 6HAJ) for (PDB: 2MHV). AFP solution structures of selected AFPs, representing three of the four AFP clades from Eurotiales: The PAF clade, including PAF (PDB: 2MHV), PAFB (PDB: 2NC2) and NFAP (PDB: 5OQS), the AFP clade, including AgAFP (PDB: 1AFP), and the BP clade, including BP (PDB: 1UOY) and PAFC (PDB: 6TRM). The structure of representatives of the NFAP2 clade has not been characterized yet. Created with BioRender.com.

Figure 5

The evolutionary conserved γ-core motif. The γ-core motif (framed in red) consists of the consensus sequence GXCX_3-9C, which can be present in the (A) dextromeric isoform (X_1-3-GXC-X_3-9-C), found in AgAFP, PAF and NFAP2 or in the levomeric isoforms (C-X_3-9-CXG-X_1-3), found in BP and PAFC. This motif is also structurally conserved, as it (B) forms the Greek letter “γ” by two antiparallel β-strands, which are connected by an interposed short turn region, shown in red in PAF (PDB: 2MHV) and PAFC (PDB: 6TRM). JALVIEW with ClustalW alignment and clustalX default colour scheme, protein structures were visualized with Chimera (Pettersen et al., 2004), α-helix is depicted in green, β-strands in blue. Created with BioRender.com.

Figure 6

Antimicrobial activity of selected AFPs of Eurotiales. Activity of representative AFPs against (A) filamentous fungi, (B) yeasts, (C) bacteria and (D) virus. Created with BioRender.com.